The present invention relates in general to the production of semiconductor devices, particularly to an electron-beam exposure process and system capable of exposing versatile patterns of semiconductor devices on an object such as a semiconductor substrate by means of a charged particle beam such as an electron beam.
Electron-beam lithography is an essential technique for fabricating advanced semiconductor integrated circuits having a large integration density. By using electron-beam lithography, it is possible to expose a pattern having a width of 0.05 .mu.m or less with an alignment error of 0.02 .mu.m or less. Therefore, it is generally considered that the electron-beam lithography will play a central role in the production of future semiconductor devices such as a DRAM (dynamic random access memory) having a large memory capacity which exceeds 256M bits or 1G bits, or very powerful high-speed microprocessors.
In the production of such advanced semiconductor devices, not only the resolution of device patterns but also the throughput at the time of exposure are of the essential importance. Since the electron-beam lithography exposes patterns by a single focused electron beam, it has a disadvantage in this regard as compared to the conventional optical exposure process which can expose all patterns in a single shot. The conventional optical exposure method, however, is reaching its limit of resolution, and therefore there are circumstances in which it is necessary to rely upon the electron-beam exposure process in the future production of high-speed/high-functional semiconductor devices.
In such situations, there are various proposals to improve the throughput of the electron-beam exposure process. For example, the applicant of the present invention proposed a so-called block exposure process. In the block exposure process, a device pattern is decomposed into a number of fundamental patterns, and the electron beam is shaped according to these fundamental patterns. The block exposure process is especially suitable for the exposure of a semiconductor memory in which a relatively small number of fundamental patterns are repeated, and has successfully achieved a throughput of 1 cm.sup.2 /second at present.
On the other hand, the block exposure process has a problem in that, in the semiconductor devices that contain complex and diversified logic circuits such as a microprocessor, the exposure efficiency is deteriorated. This is because the patterns which can be formed by the block exposure process are limited to the combination of a small number of basic, fundamental patterns. If an attempt is made to form irregular exposure patterns by the block exposure method, it is required to form a so-called variable shaped beam by activating electrostatic or electromagnetic deflectors. However, such a variable-beam shaping has a problem in that the exposure efficiency is deteriorated because of the setting time, operational speed and the like, of the deflectors.
In order to achieve the exposure of semiconductor devices containing such complex and diversified logic circuits with high throughput, there has been proposed an electron-beam exposure system having a so-called blanking aperture array (BAA) in which an exposure pattern is represented in the form of a dot pattern that contains small exposure dots. In the BAA, the electron beam that forms each dot is turned on and off depending on to the desired exposure pattern.
FIG. 1 shows a schematic view of a conventional electron-beam exposure system using a blanking aperture array. Referring to FIG. 1, the electron-beam exposure system comprises generally an electron optical system 100 that produces a focused electron beam and a control system 200 for controlling the electron optical system 100. The electron optical system 100 includes an electron gun 101 as an electron beam source, and the electron gun 101 emits the electron beam as a divergent electron beam along a predetermined optical axis O.
The electron beam thus produced by the electron gun 101 is shaped by an aperture 102a provided on an aperture plate 102, wherein the aperture 102a shapes the electron beam upon passage therethrough. The aperture 102a is in alignment with the optical axis O, and shapes the incident electron beam to have a rectangular cross section.
The shaped electron beam thus formed is focused on a BAA mask 110 by an electron lens 103, wherein the BAA mask carries thereon a blanking aperture array. Thus, the electron lens 103 projects the image of the aforementioned rectangular aperture 102a on the BAA mask 110. On the BAA mask 110, there are formed a plurality of small apertures corresponding to the exposure dots to be exposed on a semiconductor substrate, and an electrostatic deflector is provided on the BAA mask 110 in correspondence to each of the apertures. The electrostatic deflector is controlled by a driving signal E to pass the electron beam directly in a non-activated state, or to deflect the passing electron beam in an activated state, so that the direction of the passing electron beam deviates from the optical axis O. As a result, as described below, an exposure dot pattern corresponding to the non-activated apertures on the BAA mask 110 is formed on the semiconductor substrate.
The electron beam passed through the BAA mask 110 is focused at a focal point f.sub.1 on the optical axis O after passing through the electron lenses 104 and 105 that form a demagnifying optical system, and the image of the selected apertures is projected at the focal point f.sub.1. The focused electron beam is further focused on a semiconductor substrate 115 held on a movable stage 114 by electron lenses 106 and 107 that form another demagnifying optical system, after passing through a round aperture 113a provided on a round aperture plate 113. Thus, an image of the BAA mask 110 is projected on the substrate 115. Here, the electron lens 107 acts as an objective lens and includes therein various correction coils 108 and 109 for correcting focal point and aberrations as well as deflectors 111 and 112 for moving the focused electron beam over the surface of the substrate 115.
Further, there is provided an electrostatic deflector 116 between the lens 104 and lens 105, wherein the path of the electron beam is deviated from the optical axis O, which is set to pass through the round aperture 113a on the plate 113, upon activation of the electrostatic deflector 116. As a result, it becomes possible to switch the electron beam on/off at a high speed on the semiconductor substrate 115. Furthermore, the electron beams, which have been deflected by the electrostatic deflectors on the apertures on the BAA mask 110 described above, deviate also from the round aperture 113a. Therefore, the electron beams thus deflected do not reach the semiconductor substrate and it becomes possible to control the exposure dot pattern on the substrate 115.
The electron-beam exposure system of FIG. 1 uses a control system 200 for controlling such exposure operations. The control system 200 includes an external storage device 201, such as a magnetic disk device or a magnetic tape device for storing data relating to the patterns of the semiconductor device to be exposed.
The data stored in the storage device 201 is read out by a CPU 202, and the data compression thereof is removed by a data expansion circuit 203. Thereby, the data is converted to the exposure dot data which switches the individual apertures on the BAA mask 110 on/off according to the desired exposure pattern. As described later with reference to FIG. 2, the electron-beam exposure system of FIG. 1 achieves a delicate correction of the exposure pattern by conducting a multiple exposure of exposure dots on the substrate 115, wherein N independent exposure patterns are superposed. Accordingly, the data expansion circuit 203 includes N parts comprising circuits 203.sub.1 to 203.sub.N, and the circuits 203.sub.1 to 203.sub.N generate N sets of mutually independent exposure dot pattern data used for carrying out the foregoing multiple exposures superposed N times, based upon the exposure data provided from CPU 202.
Each of the circuits 203.sub.1 to 203.sub.N is composed of a buffer memory 203a for holding exposure data supplied from said CPU 202, a data expansion section 203b which generates the dot pattern data representing the exposure dot pattern based upon the exposure data held in the buffer memory 203a, and a canvas memory 203c for holding the dot pattern data expanded by the data expansion section 203b, wherein the data expansion circuit 203 supplies the dot pattern data held in the canvas memory 203c to a corresponding output buffer circuit 204. More specifically, the output buffer circuit 204 includes N holding circuits 204.sup.1 to 204.sup.N corresponding to the N data expansion circuits 203.sub.1 to 203.sub.N, and each of the holding circuits, e.g., the circuit 204.sup.1 includes 128 circuits, circuits 204.sub.1 to 204.sub.128, in correspondence to the total of 128 apertures aligned in the X direction on the BAA mask 110. Thus, each of the circuits 204.sub.1 to 204.sub.128 is supplied with and holds therein one-bit data that switches the aperture on the BAA mask 110 on/off, from said canvas memory 203c. The circuits 204.sup.1 to 204.sup.N, in turn supply the one-bit data held therein to the BAA mask 110 after converting the same into analog signals by means of corresponding D/A converters 205.sub.1 to 205.sub.N. As a result, the electrostatic deflectors aligned in the Y direction on said BAA mask 110 in correspondence to the apertures are activated sequentially.
Furthermore, the electron-beam exposure system of FIG. 1 includes an exposure control unit 206 which is supplied with a control signal from the CPU 202 based upon the control program stored in the external storage device 201, wherein the exposure control unit 206 controls the operation of the data expansion circuit 203 and the output buffer circuit 204, the transfer of data from the data expansion circuit 203 to the buffer circuit 204, and the activation of the BAA mask 110 by means of the D/A converter 205. Furthermore, the exposure control unit 206 controls the main deflector 111 and the sub deflector 112 via a main deflector control circuit 207 and a sub deflector circuit 208, such that the electron beam scans over the surface of the substrate 115. The main deflector control circuit 207 includes a distortion-correction circuit 207a for correcting aberration by driving a correction coil 109 via an astigmatic correction circuit 207b. Furthermore, the correction circuit 207a carries out the focusing correction by driving a focusing correction coil 108 via a focusing correction circuit 207c.
Furthermore, there is provided a refocus control circuit 203e in the exposure system of FIG. 1 in order to correct the dispersion of the electron beam due to the Coulomb repulsion caused at the time of focusing the electron beam. Thus, the refocus control circuit 203e adjusts properly the strength of the electron lens 106 corresponding to the exposure dot pattern.
Now, the structure of the BAA mask 110 will be briefly described with reference to FIG. 2.
Referring to FIG. 2, the BAA mask 110 carries thereon a plurality of aperture rows A.sub.1, A.sub.2, B.sub.1, B.sub.2 . . . each including 128 apertures aligned in the X direction, wherein two aperture rows that are shifted mutually by one pitch in the X and Y directions, such as the aperture rows A.sub.1 and A.sub.2, the aperture rows B.sub.1 and B.sub.2 . . . , form respective aperture groups A, B, C and D. In the illustrated example, each of the aperture groups includes only a single aperture in the Y direction (column direction), while it should be noted that these aperture groups may include a plurality of apertures aligned in the Y direction in the actual BAA mask 110. In a typical embodiment, a total of 1024 apertures are aligned in the Y direction in the aperture rows A.sub.1, B.sub.1, C.sub.1 and D.sub.1 or in the aperture rows A.sub.2, B.sub.2, C.sub.2 and D.sub.2. As the result, the BAA mask 110 forms an electron beam bundle formed of a plurality of electron beam elements aligned in rows and columns in correspondence to each of the aperture groups, and each of the electron beam elements exposes an exposure dot having typically a size of 0.05 .mu.m.times.0.05 .mu.m on the substrate 115. Thereby, exposure dots forming an array of 1024.times.128 at maximum are exposed simultaneously on the substrate 115.
The electron beam elements constituting the electron beam bundle are deflected in the Y direction of the drawing by the deflectors 111 and 112, wherein exposure dots corresponding to the aperture groups A-D are exposed on the substrate 115 repeatedly 1024 times in the maximum, in each of the locations on the substrate 115. More specifically, a row of exposure dots corresponding to the aperture row B.sub.1 is exposed over a row of exposure dots that have already been exposed on the substrate 115 in correspondence to the aperture row A.sub.1. Further, rows of the exposure dots corresponding to the aperture rows C.sub.1 and D.sub.1 are exposed in sequence thereon. A similar process is carried out with regard to the exposure of the aperture rows A.sub.2, B.sub.2, C.sub.2 and D.sub.2. Thus, after a row of exposure dots corresponding to the aperture row A.sub.2 is exposed, rows of exposure dots corresponding to the aperture rows B.sub.2, C.sub.2 and D.sub.2 are exposed thereon in sequence. Here, it should be noted that the exposure dots according to the aperture row A.sub.1 and the exposure dots according to the aperture row A.sub.2 interpolate each other to form a single dot row aligned in the X direction on the substrate 115. By shifting the apertures forming an aperture row by one pitch alternately between adjacent aperture rows in each of the aperture groups, it becomes possible to minimize the Coulomb interactions caused when the electron beam elements, shaped by the BAA mask 110, get too close to each other. If such Coulomb interactions are caused, as described above, the electron beam elements repel reciprocally to make the effective focal length of the electron lens too long.
In the BAA mask 110 of FIG. 2, it should be noted that the aperture rows in the aperture group A and the corresponding aperture rows in the adjacent aperture group B, such as the apertures forming the aperture row A.sub.1 and the apertures forming the aperture row B.sub.1, are shifted by 1/4 pitch in the X and Y directions. Similar relationships hold with regard to the aperture rows A.sub.2 and B.sub.2, B.sub.1 and C.sub.1, and B.sub.2 and C.sub.2. Generally, in the exposure mask that carries thereon N aperture groups, the apertures forming aperture rows in a first aperture group and the apertures forming corresponding aperture rows in an adjacent aperture group, are shifted with each other by M/N pitch (M&lt;N) in the X and Y direction, M being an optional integer smaller than N.
In the simplest case of the exposure using a BAA mask having such an M/N pitch shift structure, the identical exposure data is sent in sequence to the aperture rows A.sub.1, B.sub.1, C.sub.1 and D.sub.1 or to the aperture rows A.sub.2, B.sub.2, C.sub.2 and D.sub.2 to expose the dots in a desired dose of exposure so that the exposure of the desired exposure pattern is carried out. In such a BAA mask 110, it is further possible to carry out a very delicate correction of the exposure pattern by changing the exposure data depending upon each of the aperture groups. Therefore, the BAA mask 110 shown in FIG. 2 is extremely useful to correct the so-called proximity effect which causes excessive exposure, due to the electron beam being reflected or scattered on the substrate. By using the BAA mask 110 of FIG. 2, it is possible to cause the electron beam elements to scan in said Y direction simultaneously at different positions in the Y direction on the substrate 115 in correspondence to each row of the apertures A.sub.1 to D.sub. 2, whereby efficient correction of the proximity effect can be achieved.
FIG. 3 shows an example of the exposure patterns provided from the CPU 202 to the buffer memory of the data expansion section 203 such as the buffer memory 203a. Referring to FIG. 3, white circles represent the exposure dots to be formed on the substrate 115, and a set of white circles represent an exposure pattern P. In the drawing, the exposure pattern P is divided into plural cell stripes extending in the vertical direction corresponding to the Y direction of FIG. 2, while each cell stripe is formed by bit lines extending in the horizontal direction that corresponds to the X direction. Further, each of the bit lines includes 128 exposure dots aligned in the horizontal direction. Typically, the cell stripe has a size of about 100 .mu.m in the longitudinal direction and about 10 .mu.m in the transverse direction.
FIG. 4 is a diagram showing the structure of the data expansion section by taking an example of the data processing unit 203.sub.1.
Referring to FIG. 4, the exposure data provided from the external storage device 201 includes shape data OP for specifying the pattern shape, starting point data X.sub.0, Y.sub.0 for specifying the origin in X and Y directions, first size data X.sub.1 for specifying the size of the pattern in the X direction, and second size data Y.sub.1 for specifying the size of the pattern in the Y direction. The shape data OP is supplied directly from the CPU 202 to a control circuit 217 provided in the expansion section and further to a block pattern library 219 therein, while the starting point data Y.sub.0 is supplied to an address counter 203d also in the data expansion section. On the other hand, the starting point data X.sub.0 is provided to a bitmap data shifter 212, and data X.sub.1 is provided to the block pattern library 219. Thereby, the block pattern library 219 stores therein bitmap data corresponding to the data OP and X.sub.1 supplied thereto.
The bitmap data thus formed is then read out from the block pattern library 219 and is held in a register 211. Further, the bitmap data is supplied to the bitmap data shifter 212 wherein the origin of the pattern data is shifted based upon the starting point data X.sub.0. The bitmap data thus formed for one line is held in the X register 213.
Next, the bitmap data held in the X register 213 is supplied to an adder 214 and a subtracter 215, wherein the bitmap data is added to or subtracted from the bitmap data of the corresponding bit line forming the bitmap data of the exposure pattern that is already written in the canvas memory 203c. As a result, exposure dots are added or subtracted to/from the exposure pattern bitmap which has already been accumulated in the canvas memory 203c. Which one of the adder 214 or the subtracter 215 is to be used is controlled by the control circuit 217 in the data expansion section 203 based upon the shape data OP. Furthermore, according to the shape data OP, there may be the case that neither addition nor subtraction is carried out. After thus processed, the bitmap data for one bit line is supplied to an OR circuit 216 and is transferred to a temporary register 218, wherein the temporary register holds the bitmap data ready for writing into the canvas memory 203c. Further, the data in the temporary register is written into the canvas memory 203c in correspondence to the address that is specified by the address counter 203d. This address is increased with respect to the starting data Y.sub.0 described already. It should be noted that the control circuit 217 is controlled by the exposure control unit 206 of FIG. 1 and controls the canvas memory 203c, the temporary register 218, the adder 214 and the subtracter 215, based upon the shape data OP.
In the electron-beam exposure system of FIG. 1, the apertures on the BAA mask 110 shown in FIG. 2 are addressed consecutively in the Y direction, as described above. Therefore, when the dot pattern data is read out from the canvas memory 203c in the data expansion circuit 203, it is required to carry out the reading for each of the columns that are aligned in the Y direction. Here, it should be noted that the conventional canvas memory 203c uses generally a high-speed memory such as SRAM (static random access memory). In such a construction, the writing of data is achieved by specifying the Y address by the address counter 203d as shown in FIG. 4, and the data held in the temporary register 218 is written into the canvas memory 203c one line by one line with a high writing speed. It should be noted that the data for one line corresponds to one row in FIG. 2 and is written in a single writing step. In the case of reading out the dot pattern data from the canvas memory, it is also necessary to carry out the reading one line by one line by specifying Y address consecutively. In such a reading process, therefore, it is necessary to rearrange the line-by-line dot pattern data that has been read out from the canvas memory 203c into column-by-column data corresponding to the columns of exposure dots, in order to drive the BAA mask shown in FIG. 2 in correspondence to the scanning of the electron beam.
Furthermore, in such a conventional construction of the data expansion circuit 203 wherein the conventional memory is used as the canvas memory 203c, the read out operation becomes inevitably slow due to the line-by-line reading of the data that is carried out by changing the Y-address consecutively. Particularly, in the electron-beam exposure system which carries out the exposure at a high speed with a clock rate of 400 MHz or more, there is a possibility that the reading of the dot pattern from the memory becomes a bottleneck of the exposure.